The present invention relates to the field of trimming lenses, more particularly ophthalmic lenses.
An ophthalmic lens results from a succession of moulding and/or surfacing/smoothing operations determining the geometry of both convex and concave optical surfaces of said lens, followed by appropriate surface treatments
The last finishing step of an ophthalmic lens is the trimming operation consisting in machining the edge or the periphery of the glass so as to shape it according to the required dimensions to adapt the lens to the glass frame wherein it is to be arranged.
Trimming is generally done on a grinding machine comprising diamond wheels that perform the machining step as defined hereinabove.
The lens is held, upon such an operation, by axially acting blocking members.
The relative motion of the lens with respect to the grinding wheel is monitored, generally digitally, so as to provide the desired shape.
As it appears, it is mostly imperative that the lens be firmly maintained upon such a movement.
To this end, before the trimming operation, an acorn-forming step is performed on the lens, i.e. a holding means or acorn is positioned on the convex surface of the lens.
A holding pad, such a self-adhesive chip, for example a two-sided adhesive, is arranged between the acorn and the convex surface of the lens.
The so arranged lens is positioned on one of the above-mentioned axial blocking members, the second axial blocking member clamping then the lens on the concave surface thereof by means of an abutment, generally consisting in an elastomer.
Upon the machining step, a tangential torque effort is generated on the lens, which may result in a rotation of the lens relative to the acorn if the lens holding means is not sufficiently efficient.
The good holding of the lens mainly depends on the good adhesion at the interface of holding pad/convex surface of the lens.
The last generation ophthalmic lenses most often comprise an organic or mineral external layer modifying the surface energy, for example antistain hydrophobic and/or oilophobic coatings.
These are most often materials of the fluorosilane type reducing the surface energy so as to avoid the adhesion of fatty stains, which are thus easier to be removed.
Such a surface coating type may be of such an efficiency that the adhesion at the pad/convex surface interface can be thereby altered, making difficult satisfactory trimming operations, in particular for polycarbonate lenses the trimming of which generates much more large efforts in comparison with other materials.
The consequence of a badly performed trimming operation is the pure and simple loss of the lens.
That is why it is advantageous to deposit on the external layer a temporary protective layer imparting a surface energy at least equal to 15 mJ/m2, in particular a MgF2 protective layer, such as disclosed in the French Patent Application no. 0106534.
Such a method is globally satisfactory, but can be further improved. Indeed, trimming lenses having their external layer coated with a temporary MgF2 protective layer is often successfully possible only 48 hours after the lens treatment i.e. after the various layers have been deposited, in particular after the external layer and the protective layer have been deposited. When such lenses are trimmed in a period of time of less than 48 hours, the acorn-holding pad system tends to detach itself from the lens spontaneously or under a very weak effort. This is the “acorn-removing” phenomenon. Practically, this occurs when the lens is removed from the grinding machine.